The preposition gegen is usually translated as "against" in English, but there are some exceptions, especially when gegen comes up in a health context. When discussing whether a medicine is effective for a specific health condition, gegen is usually translated as "for":

BANNER PLACEHOLDER

Gegen die Halsschmerzen hilft ein Hustenbonbon.

For sore throats, a cough drop helps.

Caption 9, Krank sein - mit Eva

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Das kann ganz gut gegen das Bauchweh helfen.

This can help a lot for a stomachache.

Caption 17, Krank sein - mit Eva

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However, gegen in regards to allergies is usually translated as "to": 

Ich bin erst mal allergisch gegen Pferde geworden...

I first became allergic to horses...

Caption 44, Curly Horses - Pferdeglück auch für Allergiker

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BANNER PLACEHOLDER

One exception, however, is regarding immunization, where gegen is usually translated as "against": 

Bist du eigentlich gegen die Schweinegrippe geimpft?

Are you actually immunized against the swine flu?

Caption 24, Deutsche Musik - Thomas Godoj

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Further Learning
Do a search for gegen on Yabla German and see some more examples of how this preposition is used in German in a real world context!